The answer is B: international travel
Answer:
According to the map, the borders of the Mongol Empire increased following the death of Genghis Khan. During his lifetime, Genghis Khan was able to capture parts of northern China and large areas in Central Asia. The greatest extent of the Mongol Empire occurred in 1279
Historical themes help us study history by giving us an idea of what the time period or event being studied was about.
They accepted volunteers, but the primary way the U.S got soldiers for the war was through Selective Service aka the draft.